Wireless systems are becoming a fundamental mode of telecommunication in modern society. In order for wireless systems to continue to penetrate into the telecommunications market, the cost of providing the service must continue to decrease and the convenience of using the service should continue to increase. In response to increasing market demand, radio standards around the world have been proliferated based upon digital modulation schemes. Consequently, it is often advantageous to have a receiver that is capable of communication using more than one of these standardized techniques. In order to do so, it is necessary to have a receiver that is capable of receiving signals which have been modulated according to several different modulation techniques.
Advancement in semiconductor process technologies allows usage of oversampling bandpass delta-sigma analog-to-digital conversion in the RF frequencies, which is a new promising low-cost and reliable technique to digitize RF signals. The delta-sigma converter comprises a bandpass filter, which consists of a series of resonators in cascade, an analog-to-digital converter (A/D), that generates the converted digital output signal, and a digital-to-analog converter (D/A) that produces a plurality of analog signals converted from the digital output signal to be feed back to the resonator inputs. The first error signal is produced by the difference between the input RF signal and the first feedback signal from the D/A. A first resonator in the filter stage amplifies the first error signal to produce a more refined error signal, which is subtracted from a second feedback signal from the D/A. The sequence is repeated down the resonator stages. The output error signal from the last resonator in the bandpass filter is then sampled by the A/D. The digitized signal is converted to a feedback signal via the D/A. In order to achieve feedback stability, the sampling frequency of the A/D must be at least four times the RF signal frequency, and the digital output reproduces the high-frequency waveform of the input RF signal.
Nevertheless, oversampling an RF signal is not quite practical given the current advancement in process technologies, where the sampling clock rate may exceed tens of gigahertz. The inherent clock jitter in the sampling clock to the A/D, due to thermal agitation at the molecule level that generates phase noise in clock oscillators, severely limits the analog-to-digital conversion resolution. Also, pre-processing of the digitized RF signal requires an impractically high clock rate in the tens of gigahertz range.